Interactions of hemoglobin (Hb) with small diffusible ligands, such as O2, CO2 and NO, are known to occur at its metal centers and amino termini. The O2/CO2 delivery functions, which arise in the lung and systemic microvasculature, are allosterically controlled. Such responsiveness to the environment has not been known to apply in the case of NO. Specifically, it has been thought previously that NO does not modify the functional properties of Hb to any physiologically significant degree. Kinetic modeling predicts that the vast majority of free NO in the vasculature should be scavenged by Hb (Lancaster 1994). Accordingly, the steady-state level of NO may fall below the Km for target enzymes such as guanylate cyclase (Lancaster 1994), if not in the unperturbed organism, then with oxidant stress such as that found in atherosclerosis. These considerations raise the fundamental question of how NO exerts its biological activity.
One answer to this question is found in the propensity of nitric oxide to form S-nitrosothiols (RSNOs) (Gaston, B. et al., Proc. Natl. Acad. Sci. USA 90:10957-10961 (1993)), which retain NO-like vasorelaxant activity (Stamler, J. S., et al., Proc. Natl. Acad. Sci. USA 89:444-448 (1992)), but which can diffuse freely in and out of cells, unlike Hb. In particular, the NO group of RSNOs possesses nitrosonium (NO+) character that distinguishes it from NO itself. It is increasingly appreciated that RSNOs have the capacity to elicit certain functions that NO is incapable of (DeGroote, M. A. et al., Proc. Natl. Acad. Sci. USA 92:6399-6403 (1995); Stamler, J. S., Cell 78:931-936 (1994)). Moreover, consideration has been given to the possibility that —SNO groups in proteins serve a signaling function, perhaps analogous to phosphorylation (Stamler, J. S. et al., Proc. Natl. Acad. Sci. USA 89:444-448 (1992); Stamler, J. S. Cell, 78:931-926 (1994)). Although S-nitrosylation of proteins can regulate protein function (Stamler, J. S. et al., Proc. Natl. Acad. Sci. USA 89:444-448 (1992); Stamler, J. S., Cell, 78:931-936 (1994)), intracellular S-nitrosoproteins —the sine qua non of a regulatory posttranslational modification—has heretofore not been demonstrated.
Hemoglobin is a tetramer composed of two alpha and two beta subunits. In human Hb, each subunit contains one heme, while the beta (β) subunits also contain highly reactive SH groups (cysβ93) (Olson, J. S., Methods in Enzymology 76:631-651 (1981); Antonini, E. & Brunori, M. In Hemoglobin and Myoglobin in Their Reactions with Ligands, American Elsevier Publishing Co., Inc., New York, pp. 29-31 (1971)). These cysteine residues are highly conserved among species although their function has remained elusive.
NO (nitric oxide) is a biological “messenger molecule” which decreases blood pressure and inhibits platelet function, among other functions. NO freely diffuses from endothelium to vascular smooth muscle and platelet and across neuronal synapses to evoke biological responses. Under some conditions, reactions of NO with other components present in cells and in serum can generate toxic intermediates and products at local concentrations in tissues which are effective at inhibiting the growth of infectious organisms. Thus, it can be seen that a method of administering an effective concentration of NO or biologically active forms thereof would be beneficial in certain medical disorders.
Platelet activation is an essential component of blood coagulation and thrombotic diathesis. Activation of platelets is also seen in hematologic disorders such as sickle cell disease, in which local thrombosis is thought to be central to the painful crisis. Inhibition of platelet aggregation is therefore an important therapeutic goal in heart attacks, stroke, peripheral vascular disease and shock (disseminated intravascular coagulation). Researchers have attempted to give artificial hemoglobins to enhance oxygen delivery in all of the above disease states. However, as recently pointed out by Olsen and coworkers, administration of underivatized hemoglobin leads to platelet activation at sites of vascular injury (Olsen S. B. et al., Circulation 93:327-332 (1996)). This major problem has led experts to conclude that cell-free underivatized hemoglobins pose a significant risk of causing blood clots in the patient with vascular disease or a clotting disorder (Marcus, A. J. and J. B. Broekman, Circulation 93:208-209 (1996)). New methods of providing for an oxygen carrier and/or a method of inhibiting platelet activation would be of benefit to patients with vascular disease or who are otherwise at risk for thrombosis.